Monday, November 26, 2012

Many worlds contrary to positivism

But the key feature of the fundamentally subjective theory called quantum mechanics is that if no questions are being asked, no questions need to be answered. If the objects in our environments aren't asking any questions for us, we don't need to answer them and we don't need to imagine that the world is doing anything else than evolving the probability amplitudes according to the continuous Schrödinger's equation (or equivalent equations in other pictures). In particular, there's no "collapse" if there's no subject asking questions and learning answers! What is often called the "collapse" is the process of learning and it is a fundamentally subjective process. ...

Philosophy is never a good science. But when it comes to philosophies, many laymen in quantum mechanics – and even people not considered laymen in quantum mechanics by the society or by themselves – often think that "realism" is the right philosophy behind modern science. This viewpoint, based on millions of years of our everyday monkey-like experience, has strengthened by the 250 years of successes of classical physics and it was – unfortunately – energized by Marxism that repeated the untrue equation "science = materialist ideology" many times. Marx, Lenin, and related bastards surely belong among those who have encouraged people to never leave the mental framework of classical physics. But it is "positivism" which is the philosophy that is closest to the founders of the modern science, especially relativity and quantum mechanics.

Positivism says that all reliable knowledge – the truth we are allowed to become fans of – has to boil down to empirical observations and mathematical and logical treatments of such empirical data. It sounds uncontroversial among science types but many of them don't realize how dramatically it differs from the "materialist ideology". In particular, positivism assumes nothing about the "existence of objective reality".

I do agree with Lumo that anyone who advocates MWI (or some of the other peculiar interpretations like Bohm's) has a fundamental misunderstanding of quantum mechanics, and even of what science is all about. There are as misguided as creationists and mystics.

I don't require everyone to adopt my positivist philosophy, but the MWI advocates refuse to even acknowledge that quantum mechanics, as envisioned by Bohr and Heisenberg, is positivist.

As I have also argued, one of the big errors of MWI can be seen in terms of probability, but I disagree somewhat with how Lumo explains it. He says:

After the quantum revolution, we know that all empirical evidence coming from repeated experiments may be summarized as measured probabilities of various outcomes of diverse experiments. Once again, all the empirical knowledge about the physical processes that we have may be formulated as a collection of probabilities. Probabilities are everything we may calculate from quantum mechanics (and from other parts of science, too). So they're surely not a detail.

Orthodox quantum mechanics promotes probabilities to fundamental concepts and uses the standard probability calculus – which existed a long time before quantum mechanics – to give you rules how to verify whether the probabilistic predictions of a theory are right. The basic laws of quantum mechanics are intrinsically probabilistic.

Yes, I agree that probability is important in all of science, because it gives a tool for analyzing repeated experiments. But I do not agree that orthodox quantum mechanics promotes it to being more fundamental than that. Probability is only important in quantum mechanics to the same extent it is important in other sciences.

Nate Silver's final prediction was that Pres. Barack Obama had a 92% chance of being reelected, and Peter Norvig said 85-98%. They used reliable scientific methods to forecast the election, but we cannot say whether their probabilities were correct because we cannot directly observe the probability. (For discussion of well these predictions did, see here and here.)

The probabilities do have meaning, even tho they are not directly observable. But the probability has no meaning in MWI, because the MWI advocates would say that Mitt Romney won the election in many of the parallel universes. They could try to argue that Obama won in more universes than Romney, but there is no way to make sense out of that because we cannot see the other universes. Lumo explains this well.

Update: Lumo is enraged by this comment:

Do you consider it ironic to be attacked by some for pointing out the rationality and utility of String theories but attacked by others for pointing out the irrationality and lack of usefulness of MWI?

That hit a nerve. String theory has no utility either. It does not explain any observable phenomena, or resolve any theoretical puzzles, or simplify any physics, or have any rational justification. But Lumo is ideologically committed to string theory.

8 comments:

So it's not useful for studying strongly coupled QCD systems? It's not useful in condensed matter physics? It's not useful for studying scattering amplitudes in gauge theory and gravity? It's not useful for studying electric-magnetic duality or any other quantum field theoretic dualities? It doesn't have useful applications in pure mathematics like mirror symmetry, monstrous moonshine, and Khovanov homology?

"It does not explain any observable phenomena"

So it doesn't explain the low viscosity of the quark-gluon plasma?

"or resolve any theoretical puzzles"

So it doesn't provide an attractive solution to the black hole information paradox? It doesn't provide a concrete realization of the holographic principle and a paradigm for doing microscopic calculations of black hole entropy?

"or simplify any physics"

So it doesn't simplifiy the computation of scattering amplitudes in gauge theory?

"or have any rational justification."

It is statements like yours that do not have any rational justification. You seem to know nothing about the theory that theory that you criticize, and you fail to recognize any of its successes. String theory is not some sort of religious dogma as you and Peter Woit would have everyone believe. There are good reasons why people study string theory.

"String theory has inspired some interesting math, but the physics has been a big failure."

Huh? In the rhetorical questions of my previous post, I gave about a dozen examples where string theory is important in physics...

"As far as I know, there is no published paper that refutes what Peter Woit says about string theory. If have a reference to one, then please post it."

Well, I'm not sure what you're referring to when you talk about "what Peter Woit says" in his book. I could certainly find a paper that argues against something that he wrote, but it's rather hard to respond to "what Peter Woit says" without having a specific claim to rebut.

You brought up Woit, and denied what "Peter Woit would have everyone believe." Woit wrote a 2006 book that was a widely discussed criticism of string theory. It generated a lot of name-calling from string theorists like Motl, but the contents have not been refuted.

String theory posits that electrons are tiny strings in a 10 or 11 dimensional space with supersymmetry. The idea has fascinated a lot of smart people, but it has been a big dead-end. No Nobel prizes, for example.

Peter Woit often claims that people are studying string theory because of some sort of dogmatism, and I disagree with him about that. I think the truth is that the most spectacular results of mathematical physics of the last forty years have all come from string theory, and there are very good reasons for continuing to study it.

As far as I know, most of the historical material in Woit's book is correct. What I'm claiming is not that he got his facts all wrong, but that his conclusions about string theory are nonsensical. In order to claim that string theory has ruined physics, he defines the term "string theory" much more narrowly than most people working in the field. When Peter Woit talks about string theory, he's talking about the most speculative aspect of the subject -- the notion that elementary particles consist of tiny vibrating strings, and that this idea will unify physics and give a predictive and phenomenologically viable theory of everything.

The problem with Woit's argument is that this definition of string theory does not reflect what most string theorists are actually doing. Most string theorists are doing much more formal, mathematical work, and this work has important applications regardless of whether string theory turns out to be a successful theory of everything.

For example, string theorists have given us many important insights into the holographic nature of quantum gravity and the relationships between different quantum field theories. String theory has taught us a lot about the theories we already have, and it provides new computational techniques for making predictions with these theories. In addition, as you point out, string theory has some striking applications in pure mathematics. Because of all these applications, string theory has become established as a permanent part of the language of theoretical physics. It's not going away, and as this becomes obvious to everyone, the objections of people like Peter Woit are going to look more and more irrelevant.

Maybe we define the theory differently. I take the Wikipedia definition of String theory:

String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything (TOE), a self-contained mathematical model that describes all fundamental forces and forms of matter. String theory posits that the elementary particles (i.e., electrons and quarks) within an atom are not 0-dimensional objects, but rather 1-dimensional oscillating lines ("strings"). ... Many theoretical physicists (among them Stephen Hawking, Edward Witten, Juan Maldacena and Leonard Susskind) believe that string theory is a step towards the correct fundamental description of nature. This is because string theory allows for the consistent combination of quantum field theory and general relativity, ...

No, string theory has not been successful at any of those things. It does not describe any forces or particles. It is not a TOE or quantum gravity, or even a useful step towards those things. To the extent that it has made predictions like supersymmetry, it is contrary to experiment.

For the most part, I agree with the Wikipedia definition of string theory. For example, I think the phrase "research framework" describes quite accurately what string theory is, and the purpose is certainly to reconcile quantum mechanics and gravity. I don't think anyone would deny that it is a "contender" for a theory of everything, and I agree with the physicists listed in the article that it is a "step towards" a correct fundamental description of nature.

The problem is that some people think that string theory in its present form is supposed to be a phenomenologically realistic unified theory of physics, and that's where I disagree. If you look at the work that string theorists are doing (for example if you look at the list of talks at the annual Strings conferences), you will notice that most people are doing much more formal mathematical work. During the past fifteen years, pretty much everyone has been working on the idea of holography and the AdS/CFT correspondence. This is an extremely general principle of quantum gravity which is important regardless of whether the particles in nature are in fact made out of strings.

So when you say that string theory has not been a successful theory of quantum gravity, I have to disagree with you. String theory has resulted in extremely important insights into the holographic nature of gravity, and the importance of these insights is basically undisputed, even by people like Peter Woit. His thesis is something rather technical and, in my opinion, not particularly interesting. When Peter Woit says that string theory has failed, he's referring to something that he calls "string theory unification" -- the idea that the elementary particles are all manifestations of the same basic object, a vibrating string. I agree with him that this string unification is a speculative idea, but I think it's silly to focus on that one technical point when string theory has so many other successes.